Transducer structure that operates with acoustic waves

ABSTRACT

For improved conformance of a filter operating with acoustic waves, a transducer structure is proposed in which the separations between respectively two adjacent electrode fingers of an inter-digital transducer (IDT v ) vary over the length of the transducer. The variation thereby preferably follows a continuous distribution function. Reactance filters that are provided with such filters in the serial branch show an improved electrical conformance in the transmission range.

[0001] The invention concerns a transducer structure operating with acoustic waves, in particular for a surface wave filter (called an OFW or also a SAW filter) or an S-BAR filter (Bulk acoustic wave resonator).

[0002] In filters operating with acoustic waves, in particular in SAW filters, in particular transducer structures operating with acoustic waves (for example, SAW resonators) are used as impedance elements. Such a resonator is constructed on the surface of a piezoelectric substrate from metallic electrode structures, and comprises an inter-digital transducer with at least two connections that is normally arranged between two reflectors. Known resonators comprise inter-digital transducers that are characterized by a uniform finger period and finger width over the entire transducer. Each resonator thereby exhibits what is known as a resonance frequency and an anti-resonance frequency. The frequency position and the intensity of resonance and anti-resonance can be influenced via variation of the apertures, the finger count and the finger period. The frequency separation between resonance frequency and anti-resonance frequency, as well as its form, thereby remains the same.

[0003] In the reactance filter, the resonators are used as impedance elements and switched to an arrangement resembling a ladder (=ladder type). In addition, resonators are arranged in one serial branch and at least one, however preferably a plurality of, parallel branches. The resonance frequency of a resonator in the serial branch is set such that it approximately corresponds to the anti-resonance frequency of a resonator in the parallel branch. More complex filters, with a plurality of parallel branches and serially arranged resonators between them, can be constructed from a plurality of basic components (that respectively comprise a parallel and a serial resonator). The interaction of the resonances of the individual resonators generates a desired band-pass behavior of the filter. In addition, the resonance frequencies of the individual resonators, as well as the intensity of the resonances, are suitably set. For this purpose, finger periods, finger counts, and apertures of the individual resonators are the known degrees of freedom.

[0004] An ideal filter exhibits a good electrical conformance, a good damping behavior in the filter attenuation band, and as little insertion loss as possible in the transmission range. However, the disadvantage thereby is that the cited characteristics can mostly not be simultaneously optimized, such that only one suitable combination of characteristics can always be achieved, but not one filter optimal in all characteristics. In particular in broadband filters that exhibit a relative bandwidth of more than 2%, or in filters that are constructed on substrates with lower electro-acoustic coupling (for example, on LiTaO₃ in connection with smaller layer thickness or on quartz), an optimization only to non-optimal filters with unsatisfactory characteristics [sic].

[0005] It is the object of the present invention to specify a transducer structure operating with acoustic waves which comprises at least one further degree of freedom in the optimization, and with which, for example, a reactance filter with improved characteristics can be constructed.

[0006] This object is inventively achieved by a transducer structure according to claim 1. Advantageous embodiments of the invention, as well as advantageous uses, are to be learned from further claims.

[0007] A transducer structure comprises one or more inter-digital transducers arranged between reflectors. These again comprise electrode fingers connected with busbars that interlock like combs. In contrast to known transducers, in the inter-digital transducers of the transducer structures according to the invention, the distance between the finger centers (=finger period) per two adjacent electrode fingers is not constant, rather it varies over the entire length of the inter-digital transducer. It has been shown that a better electric conformance can be achieved in a resonator-filter environment with such a variation of the finger period. The filter can be a DMS filter, a TCF filter, or a reactance filter. In particular, the reflection at the input or, respectively, output of the filter can be minimized with a transducer structure with varying finger period. With the invention, for example, the voltage standing wave ratio (VSWR) can be reduced in a reactance filter used in the high-frequency range. Finally, with resonators according to the invention, reactance filters can be constructed that exhibit an improved passband and, in particular, an improved insertion loss.

[0008] The variation of the finger separations can be undertaken such that the precise values for the finger separations (finger periods) applied over the length of the inter-digital transducer come to lie on a curve corresponding to a continuous function. The precise values for the finger separations at a point x thus corespond [sic] to the value of the continual function scanned at the point x. Preferably, a quasi-continuous function is selected. Such a function exhibits no discontinuities.

[0009] A further advantageous variation of the finger separations is achieved when the cited spreading of the finger separations over the length of the transducer follows a function that is symmetric around an axis perpendicular to the direction of wave propagation, whereby the axis is preferably located in proximity to the center of the transducer. A function is preferably selected that exhibits a maximum at the axis of reflection.

[0010] A simple variation of the finger separations over the length of the inter-digital transducer follows a linear function in which the finger separations linearly increase or, respectively, decrease in a direction. The spreading of the finger separations can be such that the increase ensues from one end to the other end of the inter-digital transducer, or that the increase or decrease ensues up to the axis of reflection and thereafter once again decreases or, respectively, increases.

[0011] It is additionally possible to spread the finger separations over the length of the inter-digital transducer according to a nonlinear function, for example according to a parabolic function. This can also be selected such that it reaches an extreme value in the middle of the transducer that represents either a minimum or a maximum.

[0012] The finger separations are varied around an average value up to a maximum of +/−2.5%, such that a maximal difference of 5% results between two finger separations. Typically, inter-digital transducers according to the invention exhibit a maximum difference of 2 to 3%, for example of 3%. Advantageous improvements are thereby already achieved in reactance filters with smaller differences than, for example, DMS filters. The latter can fully utilize the stated variation width and exhibit a difference up to 5%.

[0013] A further variation possibility of an inventive resonator arises via an additional variation of the finger widths of the electrode fingers viewed over the length of the inter-digital transducer. This variation also preferably follows a continuous function. The variation of the finger widths can be undertaken such that the metallization proportion remains constant over the length of the transducer. However, other variations are also possible in which the metallization proportion over the length of the transducer continually increases or decreases, or in that the metallization proportion follows the corresponding distribution function of the finger separations, as the case may be a function reflected in the center of the transducer.

[0014] An inventive resonator develops with particular advantages in a reactance filter, in which it is arranged either in a parallel branch or, preferably, a serial branch. Since the form of the resonance of the resonators is changed with the inventive variation of finger widths and/or metallization proportion, in particular the passband is influenced. Since this is substantially determined via the resonators arranged in the serial branch, a maximum result is achieved with inventive resonators in the serial branch. In this manner, the electrical characteristics of the reactance filter can be changed and optimized. With the invention, a spreading of the resonance can be achieved for the individual resonator, whereby simultaneously the shape of the anti-resonance remains unchanged. In this manner, a reactance filter can be achieved which exhibits an increased bandwidth without the other characteristics of the filter simultaneously deteriorating. With the invention, reactance filters are achieved via suitable selected variations of the finger separations and finger widths that, given the same bandwidth [sic], overall show an improved electrical conformance in the transmission range of the filter.

[0015] A reactance filter with an individual inventive resonator in the serial branch already shows an improved electrical behavior. However, all resonators arranged in the serial branch are preferably provided with inventively varying finger separations and/or varying finger widths. The variation widths of the finger separations can thereby fluctuate between 0.1 and 3% around the median finger separation. In this manner, reactance filters are achieved that exhibit an increased relative bandwidth of more than 2%.

[0016] Particularly advantageous are inventive resonators or, respectively, filters produced from them constructed with poorer electro-acoustic coupling, for example on lithium tantalate with low layer thickness. Under such conditions, a better electrical conformance is achieved via an improved electro-acoustic behavior.

[0017] The invention is more closely explained in the following using an exemplary embodiment and the 8 figures belong thereto.

[0018]FIG. 1 shows a known resonator.

[0019]FIG. 2 shows a known structure for a reactance filter.

[0020]FIG. 3 shows an inventive resonator.

[0021]FIGS. 4 through 6 show inventive functions for spreading of the finger widths over the length of the transducer.

[0022]FIG. 7 compares the voltage standing wave ratio of the known and inventive reactance filters.

[0023]FIG. 8 compares the transmission range of an inventive reactance filter with that of a known reactance filter.

[0024] A resonator operating with acoustic waves in shown in FIG. 1. The metallic electrode structures (for example, consisting of aluminum, an aluminum alloy, or a multi-layer structure comprising an aluminum layer) are applied to a piezoelectric substrate. The resonator consists of an inter-digital transducer IDT that is arranged between two reflectors Ref. Each inter-digital transducer IDT consists of two telescoped comb-like electrodes that are connected with connections T1 and T2. The electrode fingers of the inter-digital transducer of known resonators exhibit a uniform electrode finger separation P that typically is defined as the distance between the finger centers of two adjacent electrode fingers. The circuit symbol for such a resonator R is shown in the right half of the image. Known possibilities to adjust the electrical properties of such a resonator R exist in the variation of the aperture A, the number N of the electrode fingers, the finger separation P of the electrode fingers, and the metallization proportion η which, viewed in the direction of wave propagation, defines the proportion of the metallized surface to the entire surface.

[0025]FIG. 2 shows an example for a reactance filter that is arranged in the form of a branch switch or, respectively, a ladder-type structure. This comprises a serial branch, in which serial resonators R_(s), are arranged, between an input IN and an output OUT. Beside this, two parallel branches are shown in the figure in which, respectively, one parallel resonator R_(p) is arranged. Typically, the parallel branch is connected to ground. However, it is also possible to symmetrically operate reactance filters. The resonators feature an impedance behavior which, given a resonance frequency, exhibit a minimal impedance; however, given an anti-resonance frequency, they exhibit a maximal impedance. Via suitable modulation of the resonance frequencies and anti-resonance frequencies of the resonators in the serial and in the parallel branches, the reactance filter fashions a passband.

[0026]FIG. 3 shows an inventive inter-digital transducer IDT_(v) with finger separation varying over the length of the inter-digital transducer. The spreading of the finger separations over the length of the transducer thereby follows a function, as it is specified in FIG. 4, for example. In an inventive transducer, the finger separations P spread over the number of fingers n lie on a preferably continuous function, in the FIG. 4, for example, on a linear. The finger separation P thereby falls over the length of the transducer from a maximum finger separation Pmax, culminating at a minimal finger separation Pmin. However, spreadings of the finger separations are also possible as they are shown in the FIGS. 5 and 6, for example. In FIG. 5, a linear spreading over the length of the transducer is likewise shown, whereby the overall distribution function is composed of two linear sub-functions that are arranged symmetric to one another in terms of an axis of reflection lying in the region of a median electrode finger Nm and vertical to the direction of wave propagation X.

[0027]FIG. 6 shows a spreading of the finger separations P that corresponds to a parabolic function whose maximum is placed here in the region of the transducer center.

[0028] In addition to the spreadings of the finger separations shown in the FIGS. 4 through 6, further functions are possible and advantageous that lead to an inventive resonator that yields improved characteristics for reactance filters manufactured therefrom. It is not required that the distribution function show a symmetry behavior as shown. However, with regards to losses it is mostly advantageous that the finger separation continually changes, that the distribution function thus exhibits no discontinuities.

[0029] As an exemplary embodiment, an inventive resonator which comprises an inter-digital transducer IDT_(v)with linear varying finger separation (as shown in FIG. 3) is used for production of a reactance filter. In addition, resonators as shown in FIG. 2 are interconnected to a reactance filter. The serial resonators R_(s) 1 through R_(s) 3 are fashioned via inventive resonators, while the parallel resonators are conventional resonators (for example, as shown in FIG. 1).

[0030] A suitable application for the invention arises, for example, in a 2in 1 filter used for mobile telephones, in which a 1 GHz reactance filter and a 2 GHz reactance filter are combined on a single substrate with uniform metallization layer thickness of, for example, 230 nm. The layer thickness of 230 nm is thereby around approximately 40% less than an optimal and typical metallization layer thickness used in 1 GHz filters. This leads in the 1 GHz filter to a clear degradation of the electrical conformance, as a consequence of which an in increased signal reflection arises at the filter input and output. This manifests in a voltage standing wave ratio VSWR of approximately 3.6, which in turn requires a correspondingly high damping of the transmitted signal in the transmission range. This unnecessarily high insertion loss generates losses whose avoidance is sought.

[0031] In FIG. 7, the gradient M2 for the voltage standing wave ratio VSWR of the specified inventive reactance filter is shown, which is contrasted to the gradient M1 of a corresponding reactance filter with conventional resonators. The gradient M1 of the reactance filter constructed with known resonators shows a maximum voltage standing wave ratio of approximately 3.6 in the transmission range, which is indicated in the upper region of the figure by a rectangle. The reactance filter provided with inventive resonators in the serial branch in contrast produces the gradient M2, which shows a clearly improved voltage standing wave ratio of approximately 2.3.

[0032] The shown transmission behavior likewise determined via comparison measurement using the function S21 is reproduced in FIG. 8. The curve D1 for the damping of the known reactance filter runs below the curve D2 for the damping behavior of the inventive reactance filter over almost the entire passband. It thus shows that the electrical conformance can be substantially improved with the invention, which in particular shows in the presented lower insertion loss of the inventive reactance filter.

[0033] For further improvement of inventive reactance filters, the metallization proportion can additionally be still set such that it is constant over the transducer, or varies corresponding to a distribution function. An optimal distribution function for the finger separations can be determined with the aid of an automatic optimization that can be implemented with software developed on the basis of electro-acoustic models. In each case, the invention shows a simple way to improve in a simple manner a poorly adapting filter, whereby previously required additional adapting elements can be dispensed with. 

1. Transducer structure operating with acoustic waves with one or more inter-digital transducers (IDT) arranged between reflectors (Ref), comprising two interlocking electrode combs with electrode fingers, in that the finger separations of an inter-digital transducer (IDT), measured between the finger centers of each two adjacent electrode fingers, varies over the length of the inter-digital transducer.
 2. Transducer structure according to claim 1, in that the variation of the finger separations over the length of the inter-digital transducer (IDT) can be specified with a continuous function.
 3. Transducer structure according to claim 1 or 2, in that the variation of the finger separations corresponds to a function that, in proximity to the center, is symmetric around an axis perpendicular to the direction of wave propagation (X).
 4. Transducer structure according to any of the claims 1 through 3, with a spreading of the finger separations over the length of the inter-digital transducer (IDT) that is conformed to a linear function.
 5. Transducer structure according to any of the claims 1 through 3, with a spreading of the finger separations over the length of the inter-digital transducer (IDT) that is conformed to a parabolic function.
 6. Transducer structure according to any of the claims 1 through 5, in that the difference between the separations of adjacent electrode fingers amounts to a maximum of 5% viewed over the entire length of the inter-digital transducer (IDT).
 7. Transducer structure according to claim 6, in that the difference amounts to a maximum of 2-3%.
 8. Transducer structure according to any of the claims 1 through 7, in that additionally the widths of the electrode fingers vary over the length of the inter-digital transducer (IDT), and their separation is conformed to a continual function.
 9. Transducer structure according to any of the claims 1 through 8, in that the finger separations are spread over the length of the inter-digital transducer (IDT) such that the finger separations in the center of the transducer are the largest.
 10. Transducer structure according to any of the claims 1 through 9, that is arranged in a reactance filter which comprises a serial and at least one parallel branch, in which respectively at least one resonator (R_(s), R_(p)) is arranged.
 11. Transducer structure according to claim 10, that is arranged in the serial branch of the reactance filter.
 12. Transducer structure according to claim 11, in that the electrode fingers of all resonators (R) arranged in the serial branch exhibit varying finger separations and/or varying finger widths.
 13. Transducer structure according to any of the claims 1 through 12, In that the variation widths of the finger separations, with regards to an average finger separation, amount to +/−0.1% to 2.5%.
 14. Use of a transducer structure according to any of the preceding claims in a reactance filter with large relative bandwidth delta f_(rel), in that 2%<delta f_(rel)<5% is valid.
 15. Use of a transducer structure according to any of the preceding claims for filters on substrates with poorer electro-acoustic coupling.
 16. Use of a transducer structure according to any of the preceding claims for filters with lower metallization depth to minimize the production spreading in the metallization. 